In a typical cellular radio system, wireless terminals (also known as mobile stations and/or user equipment units (UEs)) communicate via a radio access network (RAN) to one or more core networks. The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” (UMTS) or “eNodeB” (LTE). A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. Another identity identifying the cell uniquely in the whole mobile network is also broadcasted in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipment units (UE) within range of the base stations.
In some versions of the radio access network such as the third generation Universal Mobile Telecommunications System (UMTS), several base stations are typically connected (e.g., by landlines or microwave) to a controller node (such as a radio network controller (RNC) or a base station controller (BSC)) which supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. The 3rd Generation Partnership Project (3GPP) has developed specifications for the Evolved Universal Terrestrial Radio Access Network (E-UTRAN). The Evolved Universal Terrestrial Radio Access Network (E-UTRAN) comprises the Long Term Evolution (LTE) and System Architecture Evolution (SAE). Long Term Evolution (LTE) is a variant of a 3GPP radio access technology wherein the radio base station nodes are connected to a core network (via Access Gateways, or AGWs) rather than to radio network controller (RNC) nodes. In general, in LTE the functions of a radio network controller (RNC) node are distributed between the radio base stations nodes (eNodeB's in LTE) and AGWs. As such, the radio access network (RAN) of an LTE system has an essentially “flat” architecture comprising radio base station nodes without reporting to radio network controller (RNC) nodes.
A component of the SAE architecture is the evolved packet core (EPC), also known as the SAE core. The EPC essentially serves as an equivalent of GPRS networks through a Mobility Management Entity (MME), a serving Gateway (SGW), and PDN Gateway subcomponents. The Non Access Stratum (NAS) signaling terminates at the MME and it is also responsible for generation and allocation of temporary identities to UEs. The MME protocol stack consists of (1) a S1-MME stack to support a S1-MME interface with eNodeB, and (2) a S11 stack to support S11 interface with Serving Gateway. The MME supports the S1 interface with eNodeB. The integrated S1 MME interface stack consists of IP, SCTP, S1AP
The non-access stratum (NAS) protocols form the highest stratum of the control plane between the user equipment (UE) and MME. NAS protocols support the mobility of the UE and the session management procedures to establish and maintain IP connectivity between the UE and a packet data network gateway (PDN GW).
In cellular telecommunications, the term “handover” refers to the process of transferring an ongoing call or data session involving a wireless terminal from one node, channel, or radio access technology to another node, channel, or radio access technology. For example, a wireless terminal participating in an ongoing call or data session handled by a first or source base station may have the call or session handed over to a second or target base station when measurements from the wireless terminal indicate that it would be appropriate or beneficial for such handover to occur. However, in some instances a handover to a particular target base station or target cell may not be permitted, even if the measurements as reported by the wireless terminal seem to justify the handover. For example, the target cell may be ineligible for handover for the particular wireless terminal in view of subscription contract or subscriber agreement provisions, or other business reasons. In such sense a base station or cell may said to be “restricted” for handover purposes.
As understood from the foregoing, and interface known as the S1 interface exists between an eNodeB and a higher layer network entity such as a Mobility Management Entity (MME). Another interface, known as the X2 interface, is provided between an eNodeB and one or more neighboring eNodeB. As explained herein, some type of handovers (known as “X2 handovers”) utilize the X2 interface, whereas other types of handovers (known as “S1 handovers”) use the S1 interface.
A Handover Restriction List (HRL) is an information element which specifies one or more forbidden public land mobile networks (PLMN), one or more forbidden tracking areas (TA), one or more forbidden location areas (TA), and/or one or more other access restrictions (e.g., a forbidden inter-radio access technology (RAT)) for the wireless terminal. The standardized content of the Handover Restriction List is illustrated in Table 1. In Table 1, “M” means “mandatory” and “O” means “Optional”.
Preferably the handover restriction list (HRL) is received at a base station node upon initial UE context setup The 3GPP standard specifies a number of procedures which can be used to send a Handover Restriction List (HRL) to a base station (BS), e.g., to an eNodeB. Example 3GPP messages that may optionally contain the Handover Restriction List IE are listed as follows:                S1AP: INITIAL CONTEXT SETUP REQUEST (to request the setup of a UE context)        S1AP: HANDOVER REQUEST (to request the preparation of resources)        S1AP: DOWNLINK NAS TRANSPORT (to carry NAS information over the S1 interface)        X2AP: HANDOVER REQUEST (to request the preparation of resources for a handover).        
The Handover Restriction List information element (IE) is optionally provided to the eNodeB over the S1 interface by a Mobility Management Entity (MME) (see, e.g., reference [1]) or over the X2 interface by another eNodeB (see, e.g., reference [2]) and includes the serving public land mobile network (PLMN) and may contain equivalent PLMNs, and roaming area or access restrictions.
Upon receipt of the roaming configuration or HLR information for a user equipment unit, the eNodeB stores the received list in a UE context for the respective user equipment unit (UE). The UE context, stored in memory, comprises and/or holds subscriber information (downloaded, e.g., from a Home Subscriber Server [HSS]) as well as dynamic information such as a list of bearers that are established and capabilities of the wireless terminal. The eNodeB uses the information in a Handover Restriction List information element (IE) to determine a target cell for subsequent handover attempts. If the Handover Restriction List IE is not received, according to current convention the eNodeB considers that neither roaming area nor access restriction applies to the user equipment unit (UE).
The abovementioned standard specifications now mandate the eNodeB to reject an incoming request for handover request (2nd and 4th bullets in above list of 3GPP messages) unless the eNodeB can determine the serving public land mobile network (PLMN). In particular, the specifications state:                S1: If the eNodeB receives a HANDOVER REQUEST message which does not contain the Handover Restriction List IE, and the serving PLMN cannot be determined otherwise by the eNodeB, the eNodeB shall reject the procedure using the HANDOVER FAILURE message. If the eNodeB receives a HANDOVER REQUEST message containing the Handover Restriction List IE, and the serving PLMN indicated is not supported by the target cell, the eNodeB shall reject the procedure using the HANDOVER FAILURE message.        X2: If the target eNodeB receives a HANDOVER REQUEST message which does not contain the Handover Restriction List IE, and the PLMN to be used cannot be determined otherwise, the target eNodeB shall reject the procedure using the HANDOVER PREPARATION FAILURE message. If the target eNodeB receives a HANDOVER REQUEST message containing the Handover Restriction List IE, and the serving PLMN is not supported by the target cell, the target eNodeB shall reject the procedure using the HANDOVER PREPARATION FAILURE message.        
The references and specifications mentioned above speak of the Handover Restriction List (HRL) as being the enabler of handover restrictions in terms of forbidden tracking area identities (TAIs), location area identities (LAIs), and radio access technologies (RATs). Both the Tracking Area Identity (TAI) and location area identity (LAI) contain the public land mobile network (PLMN). But if the UE is forbidden in the PLMN, it is forbidden only locally in the tracking area or location area defined by the forbidden TAI and/or LAI.
The Handover Restriction List (HRL) also carries information which is to be interpreted as forbidden PLMNs. A forbidden public land mobile network (PLMN) is the opposite of an allowed public land mobile network (PLMN), which in turn is the sum of the serving public land mobile network (PLMN) and Equivalent public land mobile networks (PLMNs) as described by Table 1.
Usage of Tracking Area Identity (TAI), location area identity (LAI), and radio access technology (RAT) with respect to a Handover Restriction List (HRL) is relatively straightforward, since no cell which is the target for handover can support more than one of each. However, in the case of shared networks (see, e.g., reference [3]) the target cell can support several public land mobile networks (PLMNs) and there is a special meaning to/for the Handover Restriction List (HRL).
Use of the Handover Restriction List (HRL) and any associated behavior of the eNodeB which controls the target cell for the handover this fairly well established. Any base station node (e.g., eNodeB) which complies with the 3GPP standard and which is in control of a shared target cell must reject a request for handover unless the request message contains the HRL (and the PLMN to be used can not be determined otherwise by the base station note).
However, what is not described in the standard or otherwise prescribed is the relation between presence/absence of the Handover Restriction List (HRL) and any associated behavior of the eNodeB which initiates the handover, i.e., the source eNodeB which controls the source cell.
The 3GPP TS 36.413 cited as reference [1] states that “If the Handover Restriction List IE is not contained in the INITIAL CONTEXT SETUP REQUEST message, the eNodeB shall consider that neither roaming area nor access restriction applies to the UE”. The problem with the 3GPP TS 36.413 and the existing solution is that the absence of HRL suggests that any public land mobile network (PLMN) is possible. This would only rarely be the desired behavior (except if connection is an IP Multimedia Subsystem (IMS) emergency call and/or one of the E-RABs has some particular ARP value”). Operators need much more deterministic ways to control the target public land mobile network (PLMN).
Thus, when the target cell is shared between two or more PLMNs, a problem may develop since the target eNodeB must have information on exactly what PLMN to use to enable the set up of logical connection through the core network. In case of an X2 interface handover, when the signaling for the handover is executed directly between the source and target eNodeB, the source eNodeB may decide on the PLMN to be used in the target cell and explicitly specify the PLMN in the Handover Request as sent to target eNodeB. But the only possibility for the source cell to select the proper PLMN in a target cell which is shared by multiple PLMNs is by knowledge of the proper PLMN from the handover restriction list (HRL). Lacking the handover restriction list (HRL) the source cell has no possibility of selecting PLMN in the target cell, and the target cell (when receiving the Handover Request) will likely reject the handover as lacking selected PLMN.